This invention relates generally to apparatus and methods for coupling optical signals from one optical fiber into another optical fiber. More particularly, this invention relates to fiber optic couplers for coupling optical signals from one fiber into another by means of evanescent field coupling. Still more particularly, this invention relates to apparatus and methods for electrically controlling the coupling of optical signals from one optical fiber into another.
A single-mode fiber optic coupler typically is a four port device comprising a pair of optical fibers having an interaction region in which part or all of the light input by one fiber couples into the other fiber and propagates away from the interaction region. The direction of propagation into the interaction region has no effect on the amount of coupling of light between the fibers. However, light couples only in the forward direction, or direction of propagation, so that reverse coupling between adjacent ends of the two fibers does not occur.
An optical fiber comprises a core and a cladding with the refractive index of the cladding being less than that of the core. The cladding may produce a single step decrease in refractive index, or the cladding may comprise a plurality of layers of material having different indices of refraction arranged so that there are several graduated decreases in the index of refraction as the distance from the edge of the fiber core increases. In both step and graded index fibers, light propagates essentially in the core because total internal reflections at the core-cladding interface prevent light from crossing the boundary layer from the core into the cladding. However, solving Maxwell's equations and applying the appropriate well-known boundary conditions to the components of the electric field normal and tangential to the core-cladding interface gives the result that a field that is exponentially decreasing with increasing radius propagates in the cladding. Those knowledgable in the fiber optic art usually call this exponentially decreasing field the evanescent field.
It is well-known that light may couple from a first fiber into a second fiber if the second fiber is within the evanescent field of the light propagating in the first fiber. The interaction region of a single mode coupler typically includes lengths of the fiber from which part of the cladding has been removed by grinding and polishing the fibers or by chemically etching a length of intertwined fibers. When light is input to a coupler formed by placing two polished fibers adjacent one another, the amount of coupling depends upon the propagation constants of the fibers, the lengths of the interaction region and the distance between the fiber cores.
To facilitate handling the fibers and controlling the amount of cladding removed, the fibers may be mounted in a curved groove in a suitable substrate, usually formed of fused quartz. The substrate is ground away adjacent a convex outward facing portion of the fiber. The substrate and fiber are ground and polished to be optically flat. Typically the grinding rate is known, and the depth to which cladding is removed is controlled by measuring the duration of the grinding process.
All-fiber optic couplers have been available having low insertion loss. However, prior all-fiber couplers are not switchable to control the coupling of a signal from one optical fiber into another. An unswitchable fiber optic directional coupler suitable for use in single mode applications is described in Bergh, Kotler and Shaw, Electronics Letters, Vol. 16, No. 7, March, 1980, pp. 260-261. Being heretofore unswitchable, all-fiber optic couplers have seen limited use in communications systems and in other applications where switching is essential.
Another type of optical coupler employs two integrated optics parallel strip optical waveguides. Some integrated optics couplers are switchable from DC to multigigahertz rates, but such couplers have high insertion loss in the connections between the parallel strip waveguides and optical fibers. Low insertion loss is desirable to avoid unacceptable signal attenuation. Insertion loss is particularly important in switching systems in which a signal may travel through two or more couplers in succession. An integrated optics parallel strip optical coupler is described by Kogelnik and Schmidt, IEEE Journal of Quantum Electronics, Vol. QE-12, No. 7, July 1976, 396-401.